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An enantioconvergent halogenophilic nucleophilic substitution (SN2X) reaction

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Science  25 Jan 2019:
Vol. 363, Issue 6425, pp. 400-404
DOI: 10.1126/science.aau7797

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Attack from the front

Biomolecular substitution reactions are widely applied to compounds with carbon-halogen bonds. Typically, an incoming reactive group will attack the carbon from behind its bond with the halogen, causing the halogen to depart in the opposite direction. Zhang et al. now present an asymmetric catalytic substitution reaction that flips the script with an attack on the halogen from the front. Specifically, nitrogen and sulfur nucleophiles stripped bromine from a variety of carbon centers activated by electron-withdrawing groups. A chiral cationic catalyst then directed the carbon fragment back to form a carbon-sulfur or carbon-nitrogen bond enantioselectively.

Science, this issue p. 400

Abstract

Bimolecular nucleophilic substitution (SN2) plays a central role in organic chemistry. In the conventionally accepted mechanism, the nucleophile displaces a carbon-bound leaving group X, often a halogen, by attacking the carbon face opposite the C–X bond. A less common variant, the halogenophilic SN2X reaction, involves initial nucleophilic attack of the X group from the front and as such is less sensitive to backside steric hindrance. Herein, we report an enantioconvergent substitution reaction of activated tertiary bromides by thiocarboxylates or azides that, on the basis of experimental and computational mechanistic studies, appears to proceed via the unusual SN2X pathway. The proposed electrophilic intermediates, benzoylsulfenyl bromide and bromine azide, were independently synthesized and shown to be effective.

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